Mitochondrial DNA mutations activate programmed cell survival in the mouse heart

Abstract

Increased frequencies of mitochondrial DNA (mtDNA) mutations characterize the aging heart and are also found in idiopathic dilated cardiomyopathy and end-stage heart failure. The pathogenic potential of such mutations is unclear. Transgenic mice showing accelerated accumulation of mtDNA mutations and dilated cardiomyopathy due to expression of an error-prone mtDNA polymerase specifically in the heart were characterized by Western blot analysis and immunohistochemistry for the levels of pro- and antiapoptotic proteins. By 8 wk of age, when frequencies of mtDNA mutations were approximately 0.01% and all transgenic mice showed four-chamber cardiac dilation, a vigorous prosurvival response was evident. Upregulated were Bcl-2, Bcl-xl, Bfl1, heat shock protein 27, and X-linked inhibitor of apoptosis protein, all of which function to inhibit apoptosis. Although translocation of Bax to mitochondria was also seen, it was not integrated into the mitochondrial membrane. Treatment of transgenic mice with doxorubicin failed to induce apoptosis, in contrast to controls, showing that the prosurvival response protected cardiomyocytes from a death stimulus. Increased apoptosis and release of cytochrome c appeared to precede the establishment of the prosurvival state suggesting that it may reflect a response to activation of programmed cell death pathways. It has been proposed that a programmed cell survival response is activated in the failing and aging heart. We show that elevated frequencies of mtDNA mutations may serve as one trigger for the activation of such a response.

Type II diabetes increases mitochondrial DNA mutations in the left ventricle of the Goto-Kakizaki diabetic rat

“…The impact of altered mtDNA primary sequence has been more dramatically demonstrated by the mtDNA-Pol def transgenic mice in which the mtDNA polymerase lacked "proofreading" capability. In those mice, cardiac cells accumulated mtDNA mutations at a rate of more than 20-fold compared with controls, had a significant increase in apoptosis, and presented with significant heart failure (130,131). Despite this, there was not a significant change in mitochondrial function; the P/O ratio and respiratory control index were similar in mtDNAPol def and controls.…”

Decreased mtDNA, oxidative stress, cardiomyopathy, and death from transgenic cardiac targeted human mutant polymerase γ

“…3 Exonuclease activity allows for proofreading of the growing DNA strand and increases faithful copying of mtDNA. 4 Other investigators employed transgenic mouse (TG) models of defective mtDNA replication that expressed exonucleolytic proofreading-deficient Pol g and that yielded increased mtDNA mutations 5 and cardiomyopathy (CM), 6,7 but oxidative stress was not operative in those studies. 8,9 Those experimentally constructed, engineered Pol g mutations lacked a pathophysiologically based counterpart in human disease.…”

Retinoids induce differentiation and downregulate telomerase activity and N-Myc to increase sensitivity to flavonoids for apoptosis in human malignant neuroblastoma SH-SY5Y cells

“…We monitored the level of expression of COX4 (Fig. 5b) as an internal control in mitochondrial fraction (13). ß-Actin expression was examined to ensure that equal amounts of cytosolic protein were loaded in all lanes.…”